Illustrated herein in various exemplary embodiments are polymer materials containing naphthalene tetracarboxylic diimide (NTDI) dimers. The polymer materials containing NTDI dimers are capable of functioning as both a binder and an electron-transporting material in an electrophotographic element. Also illustrated herein are electrophotographic or photoconductor elements that include such polymer materials. These materials find particular application in conjunction with xerographic and electrostatographic printing processes, and will be described with particular reference thereto. It is to be appreciated, however, that the present disclosure and exemplary embodiments are also amenable to other applications.
Many electrophotographic elements currently in use are designed to be initially charged with a negative polarity. Such elements contain a material that facilitates the migration of positive holes toward the negatively charged surface in imagewise exposed areas in order to cause imagewise discharge. Such a material is often referred to as a hole-transport agent. In electrophotographic elements of that type, a positively charged toner material is usually then used to develop the remaining imagewise undischarged areas of negative polarity potential, i.e., the latent image, into a toner image. Because of the wide use of negatively charging elements, considerable numbers and types of positively charging toners have been fashioned and are available for use in electrophotographic developers.
However, for some applications of electrophotography it is more desirable to be able to develop the surface areas of the element that have been imagewise exposed to actinic radiation, rather than those that remain imagewise unexposed. For example, in laser printing of alphanumeric characters it is more desirable to be able to expose the relatively small percentage of surface area that will actually be developed to form visible alphanumeric toner images, rather than waste energy exposing the relatively large percentage of surface area that will constitute undeveloped background portions of the final image. In order to accomplish this while still employing widely available high quality positively charging toners, it is necessary to use an electrophotographic element that is designed to be positively charged. Positive charging toner can then be used to develop the exposed surface areas, which will have, after exposure and discharge, relatively negative electrostatic potential compared to the unexposed areas, where the initial positive potential will remain. An electrophotographic element designed to be initially positively charged may contain an adequate electron-transport agent, that is, a material which facilitates the migration of photogenerated electrons toward the positively charged insulative element surface.
Electrophotographic elements include both those commonly referred to as single layer, or single-active-layer, elements and those commonly referred to as multiactive, multilayer, or multi-active-layer elements.
Single-active-layer elements are so named because they contain only one layer that is active both to generate and to transport charges in response to exposure to actinic radiation. Such elements typically comprise at least an electrically conductive layer in electrical contact with an active layer. In single-active-layer elements, the active layer contains a charge-generation material to generate electron/hole pairs in response to actinic radiation and an electron-transport and/or hole-transport agent, which comprises one or more of chemical compounds capable of accepting electrons and/or holes generated by the charge-generation material and transporting them through the layer to effect discharge of the initially uniform electrostatic potential. The active layer is electrically insulative except when exposed to actinic radiation, and it sometimes contains an electrically insulative polymeric film-forming binder, which may itself be the charge-generating material, or it may be an additional material that is not charge-generating. In either case, the transport agent(s) is (are) dissolved or dispersed as uniformly as possible in the layer.
Multiactive elements are so named because they contain at least two active layers, at least one charge generation layer (CGL) which is capable of generating charges, i.e., electron/hole pairs, in response to exposure to actinic radiation, and at least one charge transport layer (CTL) which is capable of accepting and transporting charges generated by the charge-generation layer. Such elements typically comprise at least an electrically conductive layer, a CGL, and a CTL. Either the CGL or the CTL is in electrical contact with both the electrically conductive layer and the remaining CTL or CGL. The CGL contains at least a charge-generation material; the CTL contains at least a charge-transport agent; and either or both layers can contain an electrically insulative film-forming polymeric binder.
In multiactive, positively charged photoconductor elements of the type employing at least a CGL and a CTL, the CTL may be the uppermost layer of the element to protect the more mechanically sensitive CGL from wear. Known electron-transport agents may suffer from one or more problems upon repeated use, such as high dark decay, insufficient electronic charge transport activity, a gradually increasing residual potential or the like. Certain electron-transport agents, such as trinitrofluorenone (TNF), which do exhibit a useful level of sensitivity, suffer from the further disadvantage that they are now suspected to be carcinogens.
As mentioned, in both single-active-layer elements and multiactive layer elements, the transporting materials, such as electron-transport materials and hole transport materials, are typically dispersed in a polymeric binder. In particular, the transporting materials may be dispersed as a solid state solution in a polymeric binder material. Generally, device performance may be increased by increasing the concentration of the respective transport materials in a given active layer element. The concentration of the transport materials, however, is limited by the solubility of the transport materials in the binder. A single-layer photoreceptor, for example, typically comprises about 48 wt % of a polymeric binder, 30 wt % of a hole-transporting molecule, 20 wt % of an electron-transporting molecule, and 2 wt % of a charge generating material, such as a charge generating pigment. This, however, appears to be the upper limit for the concentrations of the respective components to form a solid state solution without crystallization of the transport molecules and/or loss of mechanical integrity of the device.
Thus, there is a need to provide materials that will allow for an increase in the concentrations of the transport materials in an active layer of a photoreceptor and still form a solid state solution. One way to achieve such a result would be to provide materials that have dual functionalities, i.e., function as both a binder and a transport material. U.S. Pat. Nos. 5,814,426; 5,874,192; and 5,882,814, the entire disclosures of which are incorporated herein by reference, disclose hole transport materials that also function as binder materials. U.S. Pat. No. 5,266,429 is directed to a polyesterimide that includes a dioxy component and a dicarbonyl component, one of which contains a tetracarbonyldiimide group. The polyesterimide in U.S. Pat. No. 5,266,429 may be used as a binder layer or may be the sole material in a charge transport layer.
As between electron-transporting materials and hole-transporting materials, hole-transporting materials are generally more compatible with polymeric binder materials. That is, hole-transporting materials will form a solid state solution over a wider concentration range than will electron-transport materials. Therefore it is desirable to provide a hybrid material that is capable of function as a binder material and an electron-transporting material.